Power conversion device

ABSTRACT

For power transfer from a first DC part to a second DC part in a dual active bridge (DAB) converter by stepping up a voltage, the second bridge circuit 12 includes a period in which a secondary winding n2 of an insulated transformer TR1 and the second DC part conduct and a period in which ends of the secondary winding n2 of the insulated transformer TR1 are short-circuited in the second bridge circuit 12. A control circuit 13 fixes a phase difference between a first leg a the second leg, variably controls a simultaneous off period of a fifth switching element S5 and a sixth switching element S6, and variably controls a simultaneous off period of a seventh switching element S7 and an eighth switching element S8.

TECHNICAL FIELD

The present disclosure relates to power converters that convert a DCpower into an AC power of a different voltage.

BACKGROUND ART

With the widespread use of photovoltaic power generation systems andpower storage systems, there is a demand for small-sized and highlyefficient power conditioners. In high-grade power conditioners andelectric vehicles, an insulated DC-DC converter capable of bidirectionalpower transfer and compatible with an extensive voltage range both onthe primary side and the secondary side is required. A DC-DC converterthat meets these requirements is exemplified by a dual active bridge(DAB) converter (see, for example, patent literature 1).

-   [Patent Literature 1] JP2018-166389

SUMMARY OF INVENTION Technical Problem

When a reactor of a commonly-used DAB converter according to the relatedart is charged from a DC power source on the primary side, the reactoris also charged with energy from a DC load on the secondary side, and areactive current is produced accordingly. Further, hard switching mayoccur in the presence of a light load.

The present disclosure addresses the above-described issue, and apurpose thereof is to provide a highly efficient, insulated DC-DCconverter.

Solution to Problem

A power converter according to an embodiment of the present disclosureincludes: a first bridge circuit including a first leg and a second leg,the first leg including a first switching element and a second switchingelement connected in series, the second leg including a third switchingelement and a fourth switching element connected in series, and thefirst leg and the second leg being connected in parallel to a first DCpart; a second bridge circuit including a third leg and a fourth leg,the third leg including a fifth switching element and a sixth switchingelement connected in series, the fourth leg including a seventhswitching element and an eighth switching element connected in series,and the third leg and the fourth leg being connected in parallel to asecond DC part; an insulated transformer connected between the firstbridge circuit and the second bridge circuit; a control circuit thatcontrols the first switching element—the eighth switching element.Diodes are connected or formed in antiparallel to the first switchingelement—the eighth switching element, respectively, and, for powertransfer from the first DC part to the second DC part by stepping up avoltage, the second bridge circuit includes a period in which asecondary winding of the insulated transformer and the second DC partconduct and a period in which ends of the secondary winding of theinsulated transformer are short-circuited in the second bridge circuit.The control circuit fixes a phase difference between the first leg andthe second leg, variably controls a simultaneous off period of the fifthswitching element and the sixth switching element, and variably controlsa simultaneous off period of the seventh switching element and theeighth switching element.

Advantageous Effects of Invention

According to the present disclosure, a highly efficient, insulated DC-DCconverter can be realized.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 shows a configuration of a power converter according to anembodiment;

FIGS. 2A-2F show an operation of the power converter according tocomparative example 1;

FIGS. 3A-3B show a specific example of a current flowing in the reactorin comparative example 1;

FIGS. 4A-4F show an operation of the power converter according tocomparative example 2;

FIG. 5 show a specific example of the current flowing in the reactor incomparative example 2;

FIGS. 6A-6F show an operation of the power converter according toembodiment 1 (step-down mode);

FIG. 7 shows a switching timing 1 of the first switching element S1—theeighth switching element S8 according to embodiment 1 (step-down mode);

FIG. 8 shows a switching timing 2 of the first switching element S1—theeighth switching element S8 according to embodiment 1 (step-down mode);

FIGS. 9A-9E show an operation 1 of the power converter according toembodiment 2 (step-down mode);

FIGS. 10A-10E show an operation 2 of the power converter according toembodiment 2 (step-down mode);

FIG. 11 shows a switching timing of the first switching element S1—theeighth switching element S8 according to embodiment 2 (step-down mode);

FIGS. 12A-12F show an operation of the power converter according toembodiment 1 (step-up mode);

FIG. 13 shows a switching timing 1 of the first switching element S1—theeighth switching element S8 according to embodiment 1 (step-up mode);

FIG. 14 shows a switching timing 2 of the first switching element S1—theeighth switching element S8 according to embodiment 1 (step-up mode);

FIGS. 15A-15E show an operation 1 of the power converter according toembodiment 2 (step-up mode);

FIGS. 16A-16E show an operation 2 of the power converter according toembodiment 2 (step-up mode);

FIG. 17 shows a switching timing of the first switching element S1—theeighth switching element S8 according to embodiment 2 (step-up mode);

FIG. 18 shows switching between the step-down operation and the step-upoperation of the power converter according to embodiments 1, 2; and

FIG. 19 shows a configuration of the power converter according to avariation.

DESCRIPTION OF EMBODIMENTS

FIG. 1 shows a configuration of a power converter 1 according to anembodiment. The power converter 1 is an insulated bidirectional DC-DCconverter (DAB converter) and is configured to convert a DC powersupplied from a first DC power source E1 and output a power as convertedto a second DC power source E2 or convert a DC power supplied from thesecond DC power source E2 and output a power as converted to the firstDC power source E1. The power converter 1 can transfer power by steppingdown the voltage or transfer power by stepping up the voltage.

The first DC power source E1 is embodied by, for example, a storagebattery, an electric double layer capacitor, or the like. The second DCpower source E2 is embodied by a DC bus to which a bidirectionalinverter is connected, or the like. The AC side of the bidirectionalinverter is connected to a commercial power system and an AC load inapplications of power storage systems. In applications of electricvehicles, it is connected to a motor (provided with a regenerativefunction). A DC-DC converter for solar cells or a DC-DC converter forother storage cells may further be connected to the DC bus.

The power converter 1 includes a first capacitor C1, a first bridgecircuit 11, an insulated transformer TR1, a first leak inductance L1, asecond leak inductance L2, a second bridge circuit 12, a secondcapacitor C2, and a control circuit 13.

The first capacitor C1 is coupled in parallel to the first DC powersource E1. The second capacitor C2 is coupled in parallel to the secondDC power source E2. For example, an electrolytic capacitor is used forthe first capacitor C1 and the second capacitor C2. In thisspecification, the first DC power source E1 and the first capacitor C1are collectively referred to as a first DC part, and the second DC powersource E2 and the second capacitor C2 are collectively referred to as asecond DC part.

The first bridge circuit 11 is a full-bridge circuit configured suchthat a first leg and a second leg are coupled in parallel. A firstswitching element S1 and a second switching element S2 are connected inseries in the first leg, and a third switching element S3 and a fourthswitching element S4 are connected in series in the second leg. Thefirst bridge circuit 11 is coupled in parallel to the first DC part, anda midpoint of the first leg and a midpoint of the second leg areconnected to the respective ends of the primary winding n1 of theinsulated transformer TR1.

The second bridge circuit 12 is a full-bridge circuit configured suchthat a third leg and a fourth leg are coupled in parallel. A fifthswitching element S5 and a sixth switching element S6 are connected inseries in the third leg, and a seventh switching element S7 and aneighth switching element S8 are connected in series in the fourth leg.The second bridge circuit 12 is coupled in parallel to the second DCpart, and a midpoint of the third leg and a midpoint of the fourth legare connected to the respective ends of the secondary winding n2 of theinsulated transformer TR1.

First diode D1—eighth diode D8 are connected or formed in antiparallelto the first switching element S1—the eighth switching element S8,respectively. An insulated gate bipolar transistor (IGBT) or ametal-oxide-semiconductor field-effect transistor (MOSFET) may be usedas the first switching element S1—the eighth switching element S8. Inthe case IGBTs are used, the external first diode D1—the eighth diode D8are connected to the first switching element S1—the eighth switchingelement S8, respectively. In the case MOSFETs are used, a parasiticdiode formed in the direction from the source to the drain can be usedas the first diode D1—the eighth diode D8 in the first switching elementS1—the eighth switching element S8, respectively.

The insulated transformer TR1 transforms the output voltage of the firstbridge circuit 11 connected to the primary winding n1 in accordance withthe turn ratio between the primary winding n1 and the secondary windingn2 and outputs the voltage as transformed to the second bridge circuit12 connected to the secondary winding n2. Further, the insulatedtransformer TR1 transforms the output voltage of the second bridgecircuit 12 connected to the secondary winding n2 in accordance with theturn ratio between the secondary winding n2 and the primary winding n1and outputs the voltage as converted to the first bridge circuit 11connected to the primary winding n1.

The first leak inductance L1 is formed between the midpoint of the firstleg of the first bridge circuit 11 and one end of the primary winding n1of the insulated transformer TR1. The second leak inductance L2 isformed between the third leg of the second bridge circuit 12 and one endof the secondary winding n2. A reactor element having a predeterminedinductance value may be connected in place of the first leak inductanceL1 and the second leak inductance L2.

Although not shown in FIG. 1, a first voltage sensor for detecting avoltage across the first DC part, a first current sensor for detecting acurrent flowing in the first DC part, a second voltage sensor fordetecting a voltage across the second DC part, and a second currentsensor for detecting a current flowing in the second DC part areprovided, and respective measurement values are output to the controlcircuit 13.

The control circuit 13 controls the first switching element S1—theeighth switching element S8 by suppling a driving signal (a pulse widthmodulation (PWM) signal) to the gate terminals of the first switchingelement S1—the eighth switching element S8. The configuration of thecontrol circuit 13 can be realized by cooperation of hardware resourcesand software resources or by hardware resources alone. An analog device,microcomputer, DSP, ROM, RAM, FPGA, and other LSIs can be used ashardware resources. Programs such as firmware can be used as softwareresources.

For power transfer from the first DC part to the second DC part, thecontrol circuit 13 controls the first switching element S1—the eighthswitching element S8 so that the output voltage to the second DC partmaintains the value designated by a voltage command value, based on themeasurement value of the second voltage sensor. Further, for powertransfer from the first DC part to the second DC part, the controlcircuit 13 controls the first switching element S1—the eighth switchingelement S8 so that the output current to the second DC part maintainsthe value designated by a current command value, based on themeasurement value of the second current sensor. Further, for powertransfer from the second DC part to the first DC part, the controlcircuit 13 controls the first switching element S1—the eighth switchingelement S8 so that the output voltage to the first DC part maintains thevalue designated by a voltage command value, based on the measurementvalue of the first voltage sensor. Further, for power transfer from thesecond DC part to the first DC part, the control circuit 13 controls thefirst switching element S1—the eighth switching element S8 so that theoutput current to the first DC part maintains the value designated by acurrent command value, based on the measurement value of the firstcurrent sensor.

Thus, the DAB converter is symmetrically configured on the primary sideand on the secondary side and can transfer power bidirectionally. Adescription will now be given of the operation of the power converter 1.

COMPARATIVE EXAMPLE 1

FIGS. 2A-2F show an operation of the power converter 1 according tocomparative example 1. For simplified illustration, FIGS. 2A-2F depictthe insulated transformer TR1, the first leak inductance L1, and thesecond leak inductance L2 collectively as one reactor L. Further, thefirst capacitor C1 and the second capacitor C2 are omitted from theillustration.

In the first state shown in FIG. 2A, the control circuit 13 controls thefirst switching element S1, the fourth switching element S4, the sixthswitching element S6, and the seventh switching element S7 to be in anon state and controls the second switching element S2, the thirdswitching element S3, the fifth switching element S5, and the eighthswitching element S8 to be in an off state. In the first state, thefirst DC power source E1 and the second DC power source E2 are connectedin series with the reactor L, and energy is discharged from both of thefirst DC power source E1 and the second DC power source E2 to thereactor L so as to charge the reactor L with energy.

In the second state shown in FIG. 2B, the control circuit 13 controlsthe first switching element S1, the fourth switching element S4, thefifth switching element S5, and the eighth switching element S8 to be inan on state and controls the second switching element S2, the thirdswitching element S3, the sixth switching element S6, and the seventhswitching element S7 to be in an off state. In the second state, energyis discharged from both of the first DC power source E1 and the reactorL to the second DC power source E2 so as to charge the second DC powersource E2 with energy.

In the third state shown in FIG. 2C, the control circuit 13 controls thefirst switching element S1—the eighth switching element S8 to be in anoff state. The third state is a dead time period, and a return currentflows through the second diode D2, the third diode D3, the fifth diodeD5, and the eighth diode D8. In the third state, energy is dischargedfrom the reactor L to both of the first DC power source E1 and thesecond DC power source E2 so as to charge the first DC power source E1and the second DC power source E2 with energy.

In the fourth state shown in FIG. 2D, the control circuit 13 controlsthe second switching element S2, the third switching element S3, thefifth switching element S5, and the eighth switching element S8 to be inan on state and controls the first switching element S1, the fourthswitching element S4, the sixth switching element S6, and the seventhswitching element S7 to be in an off state. In the fourth state, thefirst DC power source E1 and the second DC power source E2 are connectedin series with the reactor L, and energy is discharged from both of thefirst DC power source E1 and the second DC power source E2 to thereactor L so as to charge the reactor L with energy.

In the fifth state shown in FIG. 2E, the control circuit 13 controls thesecond switching element S2, the third switching element S3, the sixthswitching element S6, and the seventh switching element S7 to be in anon state and controls the first switching element S1, the fourthswitching element S4, the fifth switching element S5, and the eighthswitching element S8 to be in an off state. In the fifth state, energyis discharged from both of the first DC power source E1 and the reactorL to the second DC power source E2 so as to charge the second DC powersource E2 with energy.

In the sixth state shown in FIG. 2F, the control circuit 13 controls thefirst switching element S1—the eighth switching element S8 to be in anoff state. The sixth state is a dead time period, and a return currentflows through the first diode D1, the fourth diode D4, the sixth diodeD6, and the seventh diode D7. In the sixth state, energy is dischargedfrom the reactor L to both of the first DC power source E1 and thesecond DC power source E2 so as to charge the first DC power source E1and the second DC power source E2 with energy.

In comparative example 1, power is transferred from the first DC powersource E1 to the second DC power source E2 by repeating the sixswitching patterns above. In comparative example 1, the voltage orcurrent of the power transferred can be controlled by controlling aphase difference between the switching phase of the first switchingelement S1—the fourth switching element S4 and the switching phase ofthe fifth switching element S5—the eighth switching element S8.

FIGS. 3A-3B show a specific example of a current IL flowing in thereactor L in comparative example 1. FIG. 3A shows a case in which avoltage difference between the first DC power source E1 and the secondDC power source E2 is small and shows an example in which the inputvoltage of the power converter 1 is 400 V and the output voltage is 450V. FIG. 3B shows a case in which a voltage difference between the firstDC power source E1 and the second DC power source E2 is large and showsan example in which the input voltage of the power converter 1 is 200 Vand the output voltage is 450 V. In both examples, the voltage of thefirst DC power source E1 is stepped up, and the first DC power source E1charges the second DC power source E2.

In the state 1(a) and the state 4(d), the current IL flows from both ofthe first DC power source E1 and the second DC power source E2 to thereactor L so that a reactive component is produced. The second DC powersource E2 is the destination of power supply so that the energydischarged from the second DC power source E2 will return to the secondDC power source E2 afterwards. Therefore, a reactive current flows inthe reactor L from the second DC power source E2 in the state 1(a) andthe state 4(d).

In the example shown in FIG. 3B, the direction of the reactor current ILis switched in middle of the state 2(b) and in the middle of the state5(e). Switching of the positive or negative sign of the reactor currentIL in the middle reverses the direction of charging/discharging theparasitic capacitance of the switching element, and hard switchingoccurs accordingly. After the sign of the reactor current IL isswitched, the direction of power transfer will also be reversed, and areactive current is produced accordingly. The state shown in FIG. 3Balso occurs when the destination of power transfer is a light load.

COMPARATIVE EXAMPLE 2

FIGS. 4A-4F show an operation of the power converter 1 according tocomparative example 2.

In the first state shown in FIG. 4A, the control circuit 13 controls thefirst switching element S1, the fourth switching element S4, the sixthswitching element S6, and the seventh switching element S7 to be in anon state and controls the second switching element S2, the thirdswitching element S3, the fifth switching element S5, and the eighthswitching element S8 to be in an off state. In the first state, thefirst DC power source E1 and the second DC power source E2 are connectedin series with the reactor L, and energy is discharged from both of thefirst DC power source E1 and the second DC power source E2 to thereactor L so as to charge the reactor L with energy.

In the second state shown in FIG. 4B, the control circuit 13 controlsthe first switching element S1 and the fourth switching element S4 to bein an on state and controls the second switching element S2, the thirdswitching element S3, and the fifth switching element S5—the eighthswitching element S8 to be in an off state. Since all of the fifthswitching element S5—the eighth switching element S8 are in an offstate, the second bridge circuit 12 is configured as a diode bridgecircuit and functions as a rectifier circuit. In the second state,energy is discharged from both of the first DC power source E1 and thereactor L to the second DC power source E2 so as to charge the second DCpower source E2 with energy. Since the second bridge circuit 12functions as a rectifier circuit, the direction of the reactor currentIL will not be switched.

In the third state shown in FIG. 4C and the fourth state shown in FIG.4D, the control circuit 13 controls the second switching element S2, thethird switching element S3, the fifth switching element S5, and theeighth switching element S8 to be in an on state and controls the firstswitching element S1, the fourth switching element S4, the sixthswitching element S6, and the seventh switching element S7 to be in anoff state. In the third state, energy is discharged from the reactor Lto both of the first DC power source E1 and the second DC power sourceE2 so as to charge the first DC power source E1 and the second DC powersource E2 with energy. In the fourth state, the first DC power source E1and the second DC power source E2 are connected in series with thereactor L, and energy is discharged from both of the first DC powersource E1 and the second DC power source E2 to the reactor L so as tocharge the reactor L with energy. When the reactor current IL goes to 0A before a transition is made to the switching patterns shown in FIG. 4Cand FIG. 4D, a transition is not made from the state 2 to the state 3,and a transition is directly made from the state 2 to the state 4.

In the fifth state shown in FIG. 4E, the control circuit 13 controls thesecond switching element S2 and the third switching element S3 to be inan on state and controls the first switching element S1, the fourthswitching element S4, the fifth switching element S5—the eighthswitching element S8 to be in an off state. Since all of the fifthswitching element S5—the eighth switching element S8 are in an offstate, the second bridge circuit 12 is configured as a diode bridgecircuit and functions as a rectifier circuit. In the fifth state, energyis discharged from both of the first DC power source E1 and the reactorL to the second DC power source E2 so as to charge the second DC powersource E2 with energy. Since the second bridge circuit 12 functions as arectifier circuit, the direction of the reactor current IL will not beswitched.

In the sixth state shown in FIG. 4F and the first state shown in FIG.4A, the control circuit 13 controls the first switching element S1, thefourth switching element S4, the sixth switching element S6, and theseventh switching element S7 to be in an on state and controls thesecond switching element S2, the third switching element S3, the fifthswitching element S5, and the eighth switching element S8 to be in anoff state. In the sixth state, energy is discharged from the reactor Lto both of the first DC power source E1 and the second DC power sourceE2 so as to charge the first DC power source E1 and the second DC powersource E2 with energy. When the reactor current IL goes to 0 A before atransition is made to the switching patterns shown in FIG. 4F and FIG.4A, a transition is not made from the state 5 to the state 6, and atransition is directly made from the state 5 to the state 1.

In comparative example 2, power is transferred from the first DC powersource E1 to the second DC power source E2 by repeating the fourswitching patterns above. In comparative example 2, the voltage orcurrent of the power transferred can be controlled by controlling theduty ratio (on period) of the fifth switching element S5—the eighthswitching element S8. In comparative example 2, the step-up operationand the step-down operation can be switched to one another only bycontrolling the duty ratio (on period) of the fifth switching elementS5—the eighth switching element S8. Also, power can be transferredbidirectionally irrespective of the relative magnitude of the voltagesof the first DC part and the second DC part.

FIG. 5 show a specific example of the current IL flowing in the reactorL in comparative example 2. In the example shown in FIG. 5, the reactorcurrent IL goes to 0 A in the middle of the state 2(b) so that atransition is directly made from the state 2(b) to the state 4(d),bypassing the state 3(c). Similarly, the reactor current IL goes to 0 Ain the middle of the state 5(e) so that a transition is directly madefrom the state 5(e) to the state 1(a), bypassing the state 6(f).

In comparative example 2, the sign of the reactor current IL will not beswitched as shown in FIG. 3B during a period of power transmission inthe state 2(b) and in the state 5(e). This can prevent the occurrence ofhard switching and reduce loss from hard switching. However, a reactivecurrent is produced, as in the case of comparative example, 1, duringthe charging period in the state 1(a) and in the state 4(d).

Embodiment 1 (Step-Down Mode)

FIGS. 6A-6F show an operation of the power converter 1 according toembodiment 1 (step-down mode).

In the first state shown in FIG. 6A, the control circuit 13 controls thefirst switching element S1, the fourth switching element S4, and thefifth switching element S5 to be in an on state and controls the secondswitching element S2, the third switching element S3, the sixthswitching element S6, the seventh switching element S7, and the eighthswitching element S8 to be in an off state. In the first state, energyis discharged from the first DC power source E1 to both of the reactor Land the second DC power source E2 so as to charge the reactor L and thesecond DC power source E2 with energy.

In the second state shown in FIG. 6B, the control circuit 13 controlsthe second switching element S2, the fourth switching element S4, andthe eighth switching element S8 to be in an on state and controls thefirst switching element S1, the third switching element S3, the fifthswitching element S5, the sixth switching element S6, and the seventhswitching element S7 to be in an off state. In the second state, theends of the primary winding n1 of the insulated transformer TR1 areshort-circuited in the first bridge circuit 11, and the reactor L iselectrically cut off from the first DC power source E1. In the secondstate, energy is discharged from the reactor L to the second DC powersource E2 so as to charge the second DC power source E2 with energy. Theeighth switching element S8 is turned on for synchronous rectification.Synchronous rectification is useful when MOSFETs are used as theswitching elements. Even if the eighth switching element S8 is used forsynchronous rectification, the direction of the reactor current IL isnot reversed because the fifth switching element S5 is in an off state.

In the third state shown in FIG. 6C and in the fourth state shown inFIG. 6D, the control circuit 13 controls the second switching elementS2, the third switching element S3, and the sixth switching element S6to be in an on state and controls the first switching element S1, thefourth switching element S4, the fifth switching element S5, the seventhswitching element S7, and the eighth switching element S8 to be in anoff state. In the third state, energy is discharged from the reactor Lto the first DC power source E1 so as to charge the first DC powersource E1 with energy. In the fourth state, energy is discharged fromthe first DC power source E1 to both of the reactor L and the second DCpower source E2 so as to charge the reactor L and the second DC powersource E2 with energy. When the reactor current IL goes to 0 A before atransition is made to the switching patterns shown in FIG. 6C and FIG.6D, a transition is not made from the state 2 to the state 3, and atransition is directly made from the state 2 to the state 4.

In the fifth state shown in FIG. 6E, the control circuit 13 controls thefirst switching element S1, the third switching element S3, and theseventh switching element S7 to be in an on state and controls thesecond switching element S2, the fourth switching element S4, the fifthswitching element S5, the sixth switching element S6, and the eighthswitching element S8 to be in an off state. In the fifth state, the endsof the primary winding n1 of the insulated transformer TR1 areshort-circuited in the first bridge circuit 11, and the reactor L iselectrically cut off from the first DC power source E1. In the fifthstate, energy is discharged from the reactor L to the second DC powersource E2 so as to charge the second DC power source E2 with energy. Theseventh switching element S7 is turned on for synchronous rectification.Even if the seventh switching element S7 is used for synchronousrectification, the direction of the reactor current IL is not reversedbecause the sixth switching element S6 is in an off state.

In the sixth state shown in FIG. 6F and the first state shown in FIG.6A, the control circuit 13 controls the first switching element S1, thefourth switching element S4, and the fifth switching element S5 to be inan on state and controls the second switching element S2, the thirdswitching element S3, the sixth switching element S6, the seventhswitching element S7, and the eighth switching element S8 to be in anoff state. In the sixth state, energy is discharged from the reactor Lto the first DC power source E1 so as to charge the first DC powersource E1 with energy. When the reactor current IL goes to 0 A before atransition is made to the switching patterns shown in FIG. 6F and FIG.6A, a transition is not made from the state 5 to the state 6, and atransition is directly made from the state 5 to the state 1.

In embodiment 1 (step-down mode), power is transferred from the first DCpower source E1 to the second DC power source E2 by stepping down thevoltage by repeating the four switching patterns above. In embodiment 1(step-down mode), the voltage or current of the power supplied from thefirst DC part to the second DC part is controlled by controlling a phasedifference θ between the first leg (the first switching element S1 andthe second switching element S2) and the second leg (the third switchingelement S3 and the fourth switching element S4) on the primary side. Theduty ratio of the first switching element S1—the fourth switchingelement S4 is fixed at 50%.

FIG. 7 shows a switching timing 1 of the first switching element S1—theeighth switching element S8 according to embodiment 1 (step-down mode).Thin lines show on/off states of the first switching element S1, thefourth switching element S4, the fifth switching element S5, and theeighth switching element S8. Bold lines show on/off states of the secondswitching element S2, the third switching element S3, the sixthswitching element S6, and the seventh switching element S7.

The first switching element S1 and the second switching element S2operate in a complementary manner. A dead time is inserted at a point oftime when the on/off of the elements is switched. A dead time is a timeinserted to prevent the first switching element S1 and the secondswitching element S2 from conducting at the same time to short-circuitthe ends of the first DC power source E1. Similarly, the third switchingelement S3 and the fourth switching element S4 operate in acomplementary manner. A dead time is inserted at a point of time whenthe on/off of the elements is switched. The step-down rate is determinedaccording to the phase difference θ between the first switching elementS1/the second switching element S2 and the fourth switching elementS4/the third switching element S3.

In the example shown in FIGS. 6A-6F and FIG. 7, the fifth switchingelement S5 is controlled to be in an on state in the state 6(f) and thestate 1(a), and the eighth switching element S8 is controlled to be inan on state in the state 2(b). Alternatively, the eighth switchingelement S8 may be controlled to be in an on state in the state 6(f) andthe state 1(a), and the fifth switching element S5 may be controlled tobe in an on state in the state 2(b). Similarly, the sixth switchingelement S6 is controlled to be in an on state in the state 3(c) and thestate 4(d), and the seventh switching element S7 is controlled to be inan on state in the state 5(e). Alternatively, the seventh switchingelement S7 may be controlled to be in an on state in the state 3(c) andthe state 4(d), and the sixth switching element S6 may be controlled tobe in an on state in the state 5(e).

FIG. 8 shows a switching timing 2 of the first switching element S1—theeighth switching element S8 according to embodiment 1 (step-down mode).In the example shown in FIGS. 6A-6F and FIG. 7, it is described thatpower is supplied from the first DC part to the second DC part bystepping down the voltage. Alternatively, power may be supplied from thesecond DC part to the first DC part by stepping down the voltage. Inthis case, as shown in FIG. 8, the control circuit 13 may switch thedriving signal supplied to the first switching element S1—the fourthswitching element S4 and the driving signal supplied to the fifthswitching element S5—the eighth switching element S8.

As described above, embodiment 1 (step-down mode) can reduce reactivepower and improve the conversion efficiency because it does not create astate in which power is transferred from the second DC power source E2to the reactor L. In contrast, power is transferred from the second DCpower source E2 to the reactor L in the states shown in FIG. 2A, FIG. 2Daccording to comparative example 1 and in FIG. 4A, FIG. 4D according tocomparative example 2. This produces reactive power and conduction loss.According to embodiment 1 (step-down mode), the conduction loss can bereduced.

Further, synchronous rectification on the secondary side in the state2(b) and the state 5(e) can reduce conduction loss of the diode. Byusing one switching element for synchronous rectification in the state2(b) and the state 5(e), the direction of the reactor current IL isprevented from being reversed, and, at the same time, loss can bereduced. This can also prevent the occurrence of hard switching. Byproviding a short-circuit mode on the primary side, it is possible toadjust power according to a phase shift.

Embodiment 2 (Step-Down Mode)

FIGS. 9A-9E show an operation 1 of the power converter 1 according toembodiment 2 (step-down mode). FIGS. 10A-10E show an operation 2 of thepower converter 1 according to embodiment 2 (step-down mode). Embodiment2 (step-down mode) is based on embodiment 1 (step-down mode) but isconfigured to make transitions in finer steps.

In the first state shown in FIG. 9A, the control circuit 13 controls thefirst switching element S1, the fourth switching element S4, and thefifth switching element S5 to be in an on state and controls the secondswitching element S2, the third switching element S3, the sixthswitching element S6, the seventh switching element S7, and the eighthswitching element S8 to be in an off state. In the first state, energyis discharged from the first DC power source E1 to both of the reactor Land the second DC power source E2 so as to charge the reactor L and thesecond DC power source E2 with energy. This state corresponds to thefirst state in embodiment 1 (step-down mode) shown in FIG. 6A.

In the second state shown in FIG. 9B, the control circuit 13 controlsthe first switching element S1 and the fourth switching element S4 to bein an on state and controls the second switching element S2, the thirdswitching element S3, the fifth switching element S5, the sixthswitching element S6, the seventh switching element S7, and the eighthswitching element S8 to be in an off state. In the second state, too,energy is discharged from the first DC power source E1 to both of thereactor L and the second DC power source E2 so as to charge the reactorL and the second DC power source E2 with energy.

In the third state shown in FIG. 9C, the control circuit 13 controls thefourth switching element S4 and the eighth switching element S8 to be inan on state and controls the first switching element S1, the secondswitching element S2, the third switching element S3, the fifthswitching element S5, the sixth switching element S6, and the seventhswitching element S7 to be in an off state. In the third state, the endsof the primary winding n1 of the insulated transformer TR1 areshort-circuited in the first bridge circuit 11, and the reactor L iselectrically cut off from the first DC power source E1. In the thirdstate, energy is discharged from the reactor L to the second DC powersource E2 so as to charge the second DC power source E2 with energy. Theeighth switching element S8 is turned on for synchronous rectification.

In the fourth state shown in FIG. 9D, the control circuit 13 controlsthe second switching element S2, the fourth switching element S4, andthe eighth switching element S8 to be in an on state and controls thefirst switching element S1, the third switching element S3, the fifthswitching element S5, the sixth switching element S6, and the seventhswitching element S7 to be in an off state. In the fourth state, too,the ends of the primary winding n1 of the insulated transformer TR1 areshort-circuited in the first bridge circuit 11, and the reactor L iselectrically cut off from the first DC power source E1. In the fourthstate, too, energy is discharged from the reactor L to the second DCpower source E2 so as to charge the second DC power source E2 withenergy. The eighth switching element S8 is turned on for synchronousrectification. This state corresponds to the second state of embodiment1 (step-down mode) shown in FIG. 6B.

In the fifth state shown in FIG. 9E, the control circuit 13 controls thesecond switching element S2 to be in an on state and controls the firstswitching element S1, the third switching element S3, the fourthswitching element S4, the fifth switching element S5, the sixthswitching element S6, the seventh switching element S7, and the eighthswitching element S8 to be in an off state. In the fifth state, energyis discharged from the reactor L to both of the first DC power source E1and the second DC power source E2 so as to charge the first DC powersource E1 and the second DC power source E2 with energy.

In the sixth state shown in FIG. 10A, the control circuit 13 controlsthe second switching element S2, the third switching element S3, and thesixth switching element S6 to be in an on state and controls the firstswitching element S1, the fourth switching element S4, the fifthswitching element S5, the seventh switching element S7, and the eighthswitching element S8 to be in an off state. In the sixth state, energyis discharged from the first DC power source E1 to both of the reactor Land the second DC power source E2 so as to charge the reactor L and thesecond DC power source E2 with energy. This state corresponds to thefourth state of embodiment 1 (step-down mode) shown in FIG. 6D.

In the seventh state shown in FIG. 10B, the control circuit 13 controlsthe second switching element S2 and the third switching element S3 to bein an on state and controls the first switching element S1, the fourthswitching element S4, the fifth switching element S5, the sixthswitching element S6, the seventh switching element S7, and the eighthswitching element S8 to be in an off state. In the seventh state, too,energy is discharged from both of the first DC power source E1 and thereactor L to the second DC power source E2 so as to charge the second DCpower source E2 with energy.

In the eighth state shown in FIG. 10C, the control circuit 13 controlsthe third switching element S3 and the seventh switching element S7 tobe in an on state and controls the first switching element S1, secondswitching element S2, the fourth switching element S4, the fifthswitching element S5, the sixth switching element S6, and the eighthswitching element S8 to be in an off state. In the eighth state, theends of the primary winding n1 of the insulated transformer TR1 areshort-circuited in the first bridge circuit 11, and the reactor L iselectrically cut off from the first DC power source E1. In the eighthstate, energy is discharged from the reactor L to the second DC powersource E2 so as to charge the second DC power source E2 with energy. Theseventh switching element S7 is turned on for synchronous rectification.

In the ninth state shown in FIG. 10D, the control circuit 13 controlsthe first switching element S1, the third switching element S3, and theseventh switching element S7 to be in an on state and controls thesecond switching element S2, the fourth switching element S4, the fifthswitching element S5, the sixth switching element S6, and the eighthswitching element S8 to be in an off state. In the ninth state, too, theends of the primary winding n1 of the insulated transformer TR1 areshort-circuited in the first bridge circuit 11, and the reactor L iselectrically cut off from the first DC power source E1. In the ninthstate, too, energy is discharged from the reactor L to the second DCpower source E2 so as to charge the second DC power source E2 withenergy. The seventh switching element S7 is turned on for synchronousrectification. This state corresponds to the fifth state of embodiment 1(step-down mode) shown in FIG. 6E.

In the tenth state shown in FIG. 10E, the control circuit 13 controlsthe first switching element S1 to be in an on state and controls thesecond switching element S2, the third switching element S3, the fourthswitching element S4, the fifth switching element S5, the sixthswitching element S6, the seventh switching element S7, and the eighthswitching element S8 to be in an off state. In the tenth state, energyis discharged from the reactor L to both of the first DC power source E1and the second DC power source E2 so as to charge the first DC powersource E1 and the second DC power source E2 with energy.

In embodiment 2 (step-down mode), power is transferred from the first DCpower source E1 to the second DC power source E2 by stepping down thevoltage by repeating the ten switching patterns above. In embodiment 2(step-down mode), the voltage or current of the power supplied from thefirst DC part to the second DC part is controlled according to the phasedifference θ between the first leg and the second leg on the primaryside. The duty ratio of the first switching element S1—the fourthswitching element S4 is fixed at 50%. 50% is a value that does not allowfor a dead time.

FIG. 11 shows a switching timing of the first switching element S1—theeighth switching element S8 according to embodiment 2 (step-down mode).Thin lines show on/off states of the first switching element S1, thefourth switching element S4, the fifth switching element S5, and theeighth switching element S8. Bold lines show on/off states of the secondswitching element S2, the third switching element S3, the sixthswitching element S6, and the seventh switching element S7.

The first switching element S1 and the second switching element S2operate in a complementary manner. A dead time is inserted at a point oftime when the on/off of the elements is switched. Similarly, the thirdswitching element S3 and the fourth switching element S4 operate in acomplementary manner. A dead time is inserted at a point of time whenthe on/off of the elements is switched. The step-down rate is determinedaccording to the phase difference θ between the first switching elementS1/the second switching element S2 and the fourth switching elementS4/the third switching element S3.

The on period of the eighth switching element S8 and the seventhswitching element S7 is controlled to be equal to the amount of shiftcorresponding to the phase difference θ. The rising phase of the eighthswitching element S8 and the seventh switching element S7 is fixed, andthe falling phase thereof is variable.

The rising phase of the eighth switching element S8 is controlled to besynchronized with the falling phase of the first switching element S1.More specifically, the eighth switching element S8 is turned on at thesame time as the first switching element S1 is turned off. The risingphase of the seventh switching element S7 is controlled to besynchronized with the falling phase of the second switching element S2.More specifically, the seventh switching element S7 is turned on at thesame time as the second switching element S2 is turned off. Thisfacilitates zero voltage switching (ZVS) of the eighth switching elementS8 or the seventh switching element S7.

The falling phase of the eighth switching element S8 is controlled to besynchronized with the falling phase of the fourth switching element S4.More specifically, the eighth switching element S8 is turned off at thesame time as the fourth switching element S4. By causing the eighthswitching element S8 to be turned off earlier than the rising phase ofthe sixth switching element S6 by a dead time, a return current loop isprevented from being formed on the secondary side as a result of theeighth switching element S8 and the sixth switching element S6 beingturned on at the same time. The falling phase of the seventh switchingelement S7 is controlled to be synchronized with the falling phase ofthe third switching element S3. More specifically, the seventh switchingelement S7 is turned off at the same time as the third switching elementS3. By causing the seventh switching element S7 to be turned off earlierthan the rising phase of the fifth switching element S5 by a dead time,a return current loop is prevented from being formed on the secondaryside as a result of the seventh switching element S7 and the fifthswitching element S5 being turned on at the same time.

The on period of the fifth switching element S5 and the sixth switchingelement S6 is controlled to be of an amount derived from subtracting anamount of shift corresponding to the phase difference θ from the halfcycle (Ts/2) of the unit period on the primary side. The rising phase ofthe fifth switching element S5 and the sixth switching element S6 isvariable, and the falling phase thereof is fixed.

The rising phase of the fifth switching element S5 is controlled to bedelayed from the rising phase of the first switching element S1 by adead time or later. More specifically, the fifth switching element S5 isturned on when a dead time elapses since the turn-on of the firstswitching element S1 or later. The rising phase of the sixth switchingelement S6 is controlled to be delayed from the rising phase of thesecond switching element S2 by a dead time or later. More specifically,the sixth switching element S6 is turned on when a dead time elapsessince the turn-on of the second switching element S2 or later. This canreduce the occurrence of recovery loss.

The earliest rising phase of the fifth switching element S5 is delayedfrom the rising phase of the first switching element S1 by a dead time.The fifth switching element S5 does not rise earlier. Similarly, theearliest rising phase of the sixth switching element S6 is delayed fromthe rising phase of the second switching element S2 by a dead time. Thesixth switching element S6 does not rise earlier.

The falling phase of the fifth switching element S5 is controlled to beearlier than the falling phase of the first switching element S1 by adead time. More specifically, the fifth switching element S5 is turnedoff earlier than the turn-off of the first switching element S1 by adead time. This can reduce the occurrence of a reactive current from thesecondary side due to the simultaneous turn-on of the fifth switchingelement S5 and the eighth switching element S8. The falling phase of thesixth switching element S6 is controlled to be earlier than the fallingphase of the second switching element S2 by a dead time. Morespecifically, the sixth switching element S6 is turned off earlier thanthe turn-off of the second switching element S2 by a dead time. This canreduce the occurrence of a reactive current from the secondary side dueto the simultaneous turn-on of the sixth switching element S6 and theseventh switching element S7.

The phase difference θ between the first leg and the second leg on theprimary side is controlled within a range of 0 to 180°. The smaller thephase difference θ, the larger the electrical energy transferred can be.Given that the dead time is fixed, loss incurred in high-frequencyoperation can be reduced by configuring the minimum value of the phasedifference θ to be 0.

As in embodiment 1 (step-down mode), control of the fifth switchingelement S5 and control of the eighth switching element S8 may beswitched, and control of the sixth switching element S6 and control ofthe seventh switching element S7 may be switched in embodiment 2(step-down mode). It is also possible to supply power from the second DCpart to the first DC part by stepping down the voltage in embodiment 2(step-down mode), too, by switching the driving signal supplied to thefirst switching element S1—the fourth switching element S4 and thedriving signal supplied to the fifth switching element S5—the eighthswitching element S8.

As described above, embodiment 2 (step-down mode) provides the sameadvantage as embodiment 1 (step-down mode). By performing finer controlthan embodiment 1 (step-down mode), the efficiency can be furtherincreased.

Embodiment 1 (Step-Up Mode)

FIGS. 12A-12F show an operation of the power converter 1 according toembodiment 1 (step-up mode).

In the first state shown in FIG. 12A, the control circuit 13 controlsthe first switching element S1, the fourth switching element S4, and thesixth switching element S6 to be in an on state and controls the secondswitching element S2, the third switching element S3, the fifthswitching element S5, the seventh switching element S7, and the eighthswitching element S8 to be in an off state. In the first state, the endsof the secondary winding n2 of the insulated transformer TR1 areshort-circuited in the second bridge circuit 12, and the reactor L iselectrically cut off from the second DC power source E2. In the firststate, energy is discharged from the first DC power source E1 to thereactor L so as to charge the reactor L with energy.

In the second state shown in FIG. 12B, the control circuit 13 controlsthe first switching element S1, the fourth switching element S4, and theeighth switching element S8 to be in an on state and controls the secondswitching element S2, the third switching element S3, the fifthswitching element S5, the sixth switching element S6, and the seventhswitching element S7 to be in an off state. In the second state, energyis discharged from both of the first DC power source E1 and the reactorL to the second DC power source E2 so as to charge the second DC powersource E2 with energy. The eighth switching element S8 is turned on forsynchronous rectification. Even if the eighth switching element S8 isused for synchronous rectification, the direction of the reactor currentIL is not reversed because the fifth switching element S5 is in an offstate.

In the third state shown in FIG. 12C and in the fourth state shown inFIG. 12D, the control circuit 13 controls the second switching elementS2, the third switching element S3, and the fifth switching element S5to be in an on state and controls the first switching element S1, thefourth switching element S4, the sixth switching element S6, the seventhswitching element S7, and the eighth switching element S8 to be in anoff state. In the third state, energy is discharged from the reactor Lto both of the first DC power source E1 and the second DC power sourceE2 so as to charge the first DC power source E1 and the second DC powersource E2 with energy. In the fourth state, energy is discharged fromthe first DC power source E1 to the reactor L so as to charge thereactor L with energy. In the fourth state, the ends of the secondarywinding n2 of the insulated transformer TR1 are short-circuited in thesecond bridge circuit 12, and the reactor L is electrically cut off fromthe second DC power source E2. When the reactor current IL goes to 0 Abefore a transition is made to the switching patterns shown in FIG. 12Cand FIG. 12D, a transition is not made from the state 2 to the state 3,and a transition is directly made from the state 2 to the state 4.

In the fifth state shown in FIG. 12E, the control circuit 13 controlsthe second switching element S2, the third switching element S3, and theseventh switching element S7 to be in an on state and controls the firstswitching element S1, the fourth switching element S4, the fifthswitching element S5, the sixth switching element S6, and the eighthswitching element S8 to be in an off state. In the fifth state, energyis discharged from both of the first DC power source E1 and the reactorL to the second DC power source E2 so as to charge the second DC powersource E2 with energy. The seventh switching element S7 is turned on forsynchronous rectification. Even if the seventh switching element S7 isused for synchronous rectification, the direction of the reactor currentIL is not reversed because the sixth switching element S6 is in an offstate.

In the sixth state shown in FIG. 12F and in the first state shown inFIG. 12A, the control circuit 13 controls the first switching elementS1, the fourth switching element S4, and the sixth switching element S6to be in an on state and controls the second switching element S2, thethird switching element S3, the fifth switching element S5, the seventhswitching element S7, and the eighth switching element S8 to be in anoff state. In the sixth state, energy is discharged from the reactor Lto both of the first DC power source E1 and the second DC power sourceE2 so as to charge the first DC power source E1 and the second DC powersource E2 with energy. When the reactor current IL goes to 0 A before atransition is made to the switching patterns shown in FIG. 12F and FIG.12A, a transition is not made from the state 5 to the state 6, and atransition is directly made from the state 5 to the state 1.

In embodiment 1 (step-up mode), power is transferred from the first DCpower source E1 to the second DC power source E2 by stepping up thevoltage by repeating the four switching patterns above. In embodiment 1(step-up mode), the voltage or current of the power supplied from thefirst DC part to the second DC part is controlled according to a dutyratio (on period) of the fifth switching element S5—the eighth switchingelement S8 on the secondary side. Any one or more of the fifth switchingelement S5—the eighth switching element S8 may be used as the switchingelement subject to duty ratio (on period) control. The duty ratio of thefirst switching element S1—the fourth switching element S4 on theprimary side is fixed at 50%. The phase difference θ between the firstleg (the first switching element S1 and the second switching element S2)and the second leg (third switching element S3 and the fourth switchingelement S4) is fixed to be 0 or equal to or less than the dead time.

FIG. 13 shows a switching timing 1 of the first switching element S1—theeighth switching element S8 according to embodiment 1 (step-up mode).The first switching element S1 and the second switching element S2operate in a complementary manner. A dead time is inserted at a point oftime when the on/off of the elements is switched. Similarly, the thirdswitching element S3 and the fourth switching element S4 operate in acomplementary manner. A dead time is inserted at a point of time whenthe on/off of the elements is switched. The step-up rate is determinedaccording to the on period Ton of the fifth switching element S5 and thesixth switching element S6.

In the example shown in FIGS. 12A-12F and FIG. 13, the sixth switchingelement S6 is controlled to be in an on state in the state 6(f) and thestate 1(a), and the eighth switching element S8 is controlled to be inan on state in the state 2(b). Alternatively, the seventh switchingelement S7 may be controlled to be in an on state in the state 6(f) andthe state 1(a), and the fifth switching element S5 may be controlled tobe in an on state in the state 2(b). Similarly, the fifth switchingelement S5 is controlled to be in an on state in the state 3(c) and thestate 4(d), and the seventh switching element S7 is controlled to be inan on state in the state 5(e). Alternatively, the eighth switchingelement S8 may be controlled to be in an on state in the state 3(c) andthe state 4(d), and the sixth switching element S6 may be controlled tobe in an on state in the state 5(e).

FIG. 14 shows a switching timing 2 of the first switching element S1—theeighth switching element S8 according to embodiment 1 (step-up mode). Inthe example shown in FIGS. 12A-12F and FIG. 13, it is described thatpower is supplied from the first DC part to the second DC part bystepping up the voltage. Alternatively, power may be supplied from thesecond DC part to the first DC part by stepping up the voltage. In thiscase, as shown in FIG. 14, the control circuit 13 may switch the drivingsignal supplied to the first switching element S1—the fourth switchingelement S4 and the driving signal supplied to the fifth switchingelement S5—the eighth switching element S8.

As described above, embodiment 1 (step-up mode) can reduce reactivepower and improve the conversion efficiency because it does not create astate in which power is transferred from the second DC power source E2to the reactor L. In contrast, power is transferred from the second DCpower source E2 to the reactor L in the states shown in FIG. 2A, FIG. 2Daccording to comparative example 1 and in FIG. 4A, FIG. 4D according tocomparative example 2. This produces reactive power and conduction loss.According to embodiment 1 (step-up mode), power is prevented from beingtransferred from the second DC power source E2 to the reactor L, andconduction loss from a reactive current can be reduced, by providing amode in which the secondary side is short-circuited when the reactor Lis charged with energy.

Further, synchronous rectification on the secondary side in the state2(b) and the state 5(e) can reduce conduction loss of the diode. Byusing one switching element used for synchronous rectification in thestate 2(b) and the state 5(e), the direction of the reactor current ILis prevented from being reversed, and, at the same time, loss can bereduced. This can also prevent the occurrence of hard switching.

Embodiment 2 (Step-Up Mode)

FIGS. 15A-15E show an operation 1 of the power converter 1 according toembodiment 2 (step-up mode). FIGS. 16A-16E show an operation 2 of thepower converter 1 according to embodiment 2 (step-up mode). Embodiment 2(step-up mode) is based on embodiment 1 (step-up mode) but is configuredto make transitions in finer steps.

In the first state shown in FIG. 15A, the control circuit 13 controlsthe first switching element S1, the fourth switching element S4, and theseventh switching element S7 to be in an on state and controls thesecond switching element S2, the third switching element S3, the fifthswitching element S5, the sixth switching element S6, and the eighthswitching element S8 to be in an off state. In the first state, the endsof the secondary winding n2 of the insulated transformer TR1 areshort-circuited in the second bridge circuit 12, and the reactor L iselectrically cut off from the second DC power source E2. In the firststate, energy is discharged from the first DC power source E1 to thereactor L so as to charge the reactor L with energy. This statecorresponds to the first state of embodiment 1 (step-up mode) shown inFIG. 12A.

In the second state shown in FIG. 15B, the control circuit 13 controlsthe first switching element S1 and the fourth switching element S4 to bein an on state and controls the second switching element S2, the thirdswitching element S3, the fifth switching element S5, the sixthswitching element S6, the seventh switching element S7, and the eighthswitching element S8 to be in an off state. In the second state, energyis discharged from both of the first DC power source E1 and the reactorL to the second DC power source E2 so as to charge the second DC powersource E2 with energy.

In the third state shown in FIG. 15C, the control circuit 13 controlsthe first switching element S1, the fourth switching element S4, and thefifth switching element S5 to be in an on state and controls the secondswitching element S2, the third switching element S3, the sixthswitching element S6, the seventh switching element S7, and the eighthswitching element S8 to be in an off state. In the third state, too,energy is discharged from both of the first DC power source E1 and thereactor L to the second DC power source E2 so as to charge the second DCpower source E2 with energy. The fifth switching element S5 is turned onfor synchronous rectification. This state corresponds to the secondstate of embodiment 1 (step-up mode) shown in FIG. 12B.

In the fourth state shown in FIG. 15D, the control circuit 13 controlsthe first switching element S1 and the fourth switching element S4 to bein an on state and controls the second switching element S2, the thirdswitching element S3, the fifth switching element S5, the sixthswitching element S6, the seventh switching element S7, and the eighthswitching element S8 to be in an off state. In the fourth state, too,energy is discharged from both of the first DC power source E1 and thereactor L to the second DC power source E2 so as to charge the second DCpower source E2 with energy.

In the fifth state shown in FIG. 15E, the control circuit 13 controlsthe eighth switching element S8 to be in an on state and controls thefirst switching element S1, the second switching element S2, the thirdswitching element S3, the fourth switching element S4, the fifthswitching element S5, the sixth switching element S6, and the seventhswitching element S7 to be in an off state. In the fifth state, energyis discharged from the reactor L to both of the first DC power source E1and the second DC power source E2 so as to charge the first DC powersource E1 and the second DC power source E2 with energy.

In the sixth state shown in FIG. 16A, the control circuit 13 controlsthe second switching element S2, the third switching element S3, and theeighth switching element S8 to be in an on state and controls the firstswitching element S1, the fourth switching element S4, the fifthswitching element S5, the sixth switching element S6, and the seventhswitching element S7 to be in an off state. In the sixth state, the endsof the secondary winding n2 of the insulated transformer TR1 areshort-circuited in the second bridge circuit 12, and the reactor L iselectrically cut off from the second DC power source E2. In the sixthstate, energy is discharged from the first DC power source E1 to thereactor L so as to charge the reactor L with energy. This statecorresponds to the fourth state of embodiment 1 (step-up mode) shown inFIG. 12D.

In the seventh state shown in FIG. 16B, the control circuit 13 controlsthe second switching element S2 and the third switching element S3 to bein an on state and controls the first switching element S1, the fourthswitching element S4, the fifth switching element S5, the sixthswitching element S6, the seventh switching element S7, and the eighthswitching element S8 to be in an off state. In the seventh state, energyis discharged from both of the first DC power source E1 and the reactorL to the second DC power source E2 so as to charge the second DC powersource E2 with energy.

In the eighth state shown in FIG. 16C, the control circuit 13 controlsthe second switching element S2, the third switching element S3, and thesixth switching element S6 to be in an on state and controls the firstswitching element S1, the fourth switching element S4, the fifthswitching element S5, the seventh switching element S7, and the eighthswitching element S8 to be in an off state. In the eighth state, too,energy is discharged from both of the first DC power source E1 and thereactor L to the second DC power source E2 so as to charge the second DCpower source E2 with energy. The sixth switching element S6 is turned onfor synchronous rectification. This state corresponds to the fifth stateof embodiment 1 (step-up mode) shown in FIG. 12E.

In the ninth state shown in FIG. 16D, the control circuit 13 controlsthe second switching element S2 and the third switching element S3 to bein an on state and controls the first switching element S1, the fourthswitching element S4, the fifth switching element S5, the sixthswitching element S6, the seventh switching element S7, and the eighthswitching element S8 to be in an off state. In the ninth state, too,energy is discharged from both of the first DC power source E1 and thereactor L to the second DC power source E2 so as to charge the second DCpower source E2 with energy.

In the tenth state shown in FIG. 16E, the control circuit 13 controlsthe seventh switching element S7 to be in an on state and controls thefirst switching element S1, the second switching element S2, the thirdswitching element S3, the fourth switching element S4, the fifthswitching element S5, the sixth switching element S6, and the eighthswitching element S8 to be in an off state. In the tenth state, energyis discharged from the reactor L to both of the first DC power source E1and the second DC power source E2 so as to charge the first DC powersource E1 and the second DC power source E2 with energy.

In embodiment 2 (step-up mode), power is transferred from the first DCpower source E1 to the second DC power source E2 by stepping up thevoltage by repeating the ten switching patterns above. In embodiment 2(step-up mode), the voltage or current of the power supplied from thefirst DC part to the second DC part is controlled according to a dutyratio (on period) of the eighth switching element S8 and the seventhswitching element S7 on the secondary side. The duty ratio of the firstswitching element S1—the fourth switching element S4 on the primary sideis fixed at 50%. 50% is a value that does not allow for a dead time. Thephase difference θ between the first leg and the second leg on theprimary side is fixed at 0.

FIG. 17 shows a switching timing of the first switching element S1—theeighth switching element S8 according to embodiment 2 (step-up mode).Thin lines show on/off states of the first switching element S1, thefourth switching element S4, the fifth switching element S5, and theeighth switching element S8. Bold lines show on/off states of the secondswitching element S2, the third switching element S3, the sixthswitching element S6, and the seventh switching element S7.

The first switching element S1 and the second switching element S2operate in a complementary manner. A dead time is inserted at a point oftime when the on/off of the elements is switched. Similarly, the thirdswitching element S3 and the fourth switching element S4 operate in acomplementary manner. A dead time is inserted at a point of time whenthe on/off of the elements is switched. The step-up rate is determinedaccording to the on period Ton of the eighth switching element S8 andthe seventh switching element S7.

The on period Ton of the eighth switching element S8 and the seventhswitching element S7 is controlled according to the duty. The risingphase of the eighth switching element S8 and the seventh switchingelement S7 is fixed, and the falling phase thereof is variable.

The rising phase of the eighth switching element S8 is controlled to besynchronized with the falling phase of the first switching element S1.More specifically, the eighth switching element S8 is turned on at thesame time as the first switching element S1 is turned off. The risingphase of the seventh switching element S7 is controlled to besynchronized with the falling phase of the second switching element S2.More specifically, the seventh switching element S7 is turned on at thesame time as the second switching element S2 is turned off. Thisfacilitates ZVS of the eighth switching element S8 or the seventhswitching element S7.

The on period of the fifth switching element S5 and the sixth switchingelement S6 is controlled to be of an amount derived from subtracting anamount of shift corresponding to the on period Ton of the eighthswitching element S8 and the seventh switching element S7 from the halfcycle (Ts/2) of the unit period on the primary side. The rising phase ofthe fifth switching element S5 and the sixth switching element S6 isvariable, and the falling phase thereof is fixed.

The rising phase of the fifth switching element S5 is controlled to bedelayed from the rising phase of the first switching element S1 by adead time or later. More specifically, the fifth switching element S5 isturned on when a dead time elapses since the turn-on of the firstswitching element S1 or later. The rising phase of the sixth switchingelement S6 is controlled to be delayed from the rising phase of thesecond switching element S2 by a dead time or later. More specifically,the sixth switching element S6 is turned on when a dead time elapsessince the turn-on of the second switching element S2 or later. This canreduce the occurrence of recovery loss.

The earliest rising phase of the fifth switching element S5 is delayedfrom the rising phase of the first switching element S1 by a dead time.The fifth switching element S5 does not rise earlier. Similarly, theearliest rising phase of the sixth switching element S6 is delayed fromthe rising phase of the second switching element S2 by a dead time. Thesixth switching element S6 does not rise earlier.

The falling phase of the fifth switching element S5 is controlled to beearlier than the falling phase of the first switching element S1 by adead time. More specifically, the fifth switching element S5 is turnedoff earlier than the turn-off of the first switching element S1 by adead time. This can reduce the occurrence of a reactive current from thesecondary side due to the simultaneous turn-on of the fifth switchingelement S5 and the eighth switching element S8. The falling phase of thesixth switching element S6 is controlled to be earlier than the fallingphase of the second switching element S2 by a dead time. Morespecifically, the sixth switching element S6 is turned off earlier thanthe turn-off of the second switching element S2 by a dead time. This canreduce the occurrence of a reactive current from the secondary side dueto the simultaneous turn-on of the sixth switching element S6 and theseventh switching element S7.

The electrical energy transferred is controlled according to the onperiod Ton of the eighth switching element S8 and the seventh switchingelement S7. The longer the on period Ton, the larger the electricalenergy transferred can be. Given that the dead time is fixed, lossincurred in high-frequency operation can be reduced by configuring thephase difference between the first leg and the second leg on the primaryside to be 0.

As in embodiment 1 (step-up mode), control of the fifth switchingelement S5 and control of the eighth switching element S8 may beswitched, and control of the sixth switching element S6 and control ofthe seventh switching element S7 may be switched in embodiment 2(step-up mode). It is also possible to supply power from the second DCpart to the first DC part by stepping up the voltage in embodiment 2(step-up mode), too, by switching the driving signal supplied to thefirst switching element S1—the fourth switching element S4 and thedriving signal supplied to the fifth switching element S5—the eighthswitching element S8.

As described above, embodiment 2 (step-up mode) provides the sameadvantage as embodiment 1 (step-up mode). By performing finer controlthan embodiment 1 (step-up mode), the efficiency can be furtherincreased.

In comparative example 2 described above, it is possible to switchbetween the step-down operation and the step-up operation by controllingthe duty ratio (on period) of the fifth switching element S5—the eighthswitching element S8 on the secondary side. In embodiments 1 and 2, onthe other hand, the phase difference θ between the first leg and thesecond leg is controlled in the step-down mode, and the duty ratio (onperiod) of the fifth switching element S5—the eighth switching elementS8 on the secondary side is controlled in the step-up mode.

For power transfer from the first DC part to the second DC part, thecontrol circuit 13 switches between the step-down mode and the step-upmode based on the voltage of the first DC part and the voltage of thesecond DC part. When the voltage of the second DC part is lower than thevoltage of the first DC part, the control circuit 13 selects thestep-down mode. When the voltage of the second DC part is higher thanthe voltage of the first DC part, the control circuit 13 selects thestep-up mode. Further, for power transfer from the second DC part to thefirst DC part, the control circuit 13 switches between the step-downmode and the step-up mode based on the voltage of the second DC part andthe voltage of the first DC part. When the voltage of the first DC partis lower than the voltage of the second DC part, the control circuit 13selects the step-down mode. When the voltage of the first DC part ishigher than the voltage of the second DC part, the control circuit 13selects the step-up mode. The control circuit 13 may switch between thestep-down mode and the step-up mode based on the direction of thecurrent flowing in the first DC part, the direction of the currentflowing in the second DC part, or the direction of the reactor currentIL.

FIG. 18 shows switching between the step-down operation and the step-upoperation of the power converter 1 according to embodiments 1, 2. In thestep-down operation, the on period Ton of the fifth switching elementS5—the eighth switching element S8 on the secondary side is fixed to beequal to or less than a dead time Td (which could be 0), and the phasedifference θ between the first leg and the second leg on the primaryside is controlled. In the step-up operation, the phase difference θbetween the first leg and the second leg on the primary side is fixed tobe equal to or less than the dead time Td (which could be 0), and the onperiod Ton of the fifth switching element S5—the eighth switchingelement S8 on the secondary side is controlled.

The maximum value of the phase difference θ and the maximum value of theon period are both the half cycle (Ts/2). Since the phase differenceθ/the on period Ton used when the maximum power is output in thestep-down operation and the phase difference θ/on period Ton used whenthe maximum power is output in the step-up operation are equal, seamlessswitching between the step-down operation and the step-up operation ispossible.

As described above, the combination of the step-down mode and thestep-up mode according to embodiments 1, 2 makes it possible to use oneDC-DC converter to perform the step-down operation and the step-upoperation and transfer power bidirectionally. Accordingly, the convertercan be compatible with an extensive voltage range both on the primaryside and the secondary side.

Described above is an explanation of the present disclosure based on theembodiment. The embodiment is intended to be illustrative only and itwill be understood by those skilled in the art that variousmodifications to constituting elements and processes could be developedand that such modifications are also within the scope of the presentdisclosure.

In embodiment 1 (step-down mode), the eighth switching element S8 iscontrolled to be in an on state in the state 2(b), and the seventhswitching element S7 is controlled to be in an on state in the state5(e), as shown in FIGS. 6A-6F, for synchronous rectification.Alternatively, synchronous rectification in the state 2(b) and the state5(e) may be omitted. More specifically, the fifth switching elementS5—the eighth switching element S8 may all be controlled to be in an offstate in the state 2(b) and the state 5(e).

In embodiment 1 (step-up mode), the eighth switching element S8 iscontrolled to be in an on state in the state 2(b), and the seventhswitching element S7 is controlled to be in an on state in the state5(e), as shown in FIGS. 12A-12F, for synchronous rectification.Alternatively, synchronous rectification in the state 2(b) and the state5(e) may be omitted. More specifically, the fifth switching elementS5—the eighth switching element S8 may all be controlled to be in an offstate in the state 2(b) and the state 5(e).

FIG. 19 shows a configuration of the power converter 1 according to avariation. The power converter 1 according to the variation is aninsulated unidirectional DC-DC converter. It can be used in applicationsin which the first DC power source 1 on the primary side is not chargedby the load R2 on the secondary side. In the power converter 1 accordingto the variation, two diode devices (the seventh diode D7 and the eighthdiode D8) are used in place of the seventh switching element S7 and theeighth switching element S8 in the second bridge circuit 12.

When synchronous rectification is omitted in the state 2(b) and thestate 5(e) in the step-up mode shown in FIGS. 12A-12F and FIG. 13, theseventh switching element S7 and the eighth switching element S8 are ina continuous off state. In this case, the step-up operation is equallypossible in the power converter 1 according to the variation byperforming the same control as performed in embodiment 1. By omittingsynchronous rectification in the state 2(b) and the state 5(e) in thestep-down mode shown in FIGS. 6A-6F and FIG. 7, the step-down operationis equally possible in the power converter 1 according to the variationby performing the same control as performed in embodiment 1. Accordingto the variation, the cost of the second bridge circuit 12 can bereduced.

The embodiments may be defined by the following items.

[Item 1]

1. A power converter (1) including:

a first bridge circuit (11) including a first leg and a second leg, thefirst leg including a first switching element (S1) and a secondswitching element (S2) connected in series, the second leg including athird switching element (S3) and a fourth switching element (S4)connected in series, and the first leg and the second leg beingconnected in parallel to a first DC part (E1, C1);

a second bridge circuit (12) including a third leg and a fourth leg, thethird leg including a fifth switching element (S5) and a sixth switchingelement (S6) connected in series, the fourth leg including a seventhswitching element (S7) and an eighth switching element (S8) connected inseries, and the third leg and the fourth leg being connected in parallelto a second DC part (C2, E2);

an insulated transformer (TR1) connected between the first bridgecircuit (11) and the second bridge circuit (12);

a control circuit (13) that controls the first switching element(S1)—the eighth switching element (S8), wherein

diodes (D1-D8) are connected or formed in antiparallel to the firstswitching element (S1)—the eighth switching element (S8), respectively,and,

for power transfer from the first DC part (E1, C1) to the second DC part(C2, E2) by stepping up a voltage,

the second bridge circuit (12) includes a period in which a secondarywinding (n2) of the insulated transformer (TR1) and the second DC part(C2, E2) conduct and a period in which ends of the secondary winding(n2) of the insulated transformer (TR1) are short-circuited in thesecond bridge circuit (12),

the control circuit (13)

fixes a phase difference between the first leg and the second leg,

variably controls a simultaneous off period of the fifth switchingelement (S5) and the sixth switching element (S6), and

variably controls a simultaneous off period of the seventh switchingelement (S7) and the eighth switching element (S8).

This realizes a highly efficient DC-DC converter of step-up type inwhich a reactive current is reduced.

[Item 2]

The power converter (1) according to item 1, wherein

the control circuit (13) performs control that includes:

a first pattern in which the first switching element (S1) and the fourthswitching element (S4) are in an on state, the second switching element(S2) and the third switching element (S3) are in an off state, and endsof the secondary winding (n2) of the insulated transformer (TR1) areshort-circuited in the second bridge circuit (12);

a second pattern in which the first switching element (S1) and thefourth switching element (S4) are in an on state, the second switchingelement (S2) and the third switching element (S3) are in an off state,and the second bridge circuit (12) is in a rectification state;

a third pattern in which the second switching element (S2) and the thirdswitching element (S3) are in an on state, the first switching element(S1) and the fourth switching element (S4) are in an off state, and theends of the secondary winding (n2) of the insulated transformer (TR1)are short-circuited in the second bridge circuit (12); and

a fourth pattern in which the second switching element (S2) and thethird switching element (S3) are in an on state, the first switchingelement (S1) and the fourth switching element (S4) are in an off state,and the second bridge circuit (12) is in a rectification state.

This realizes a highly efficient DC-DC converter of step-up type inwhich a reactive current is reduced.

[Item 3]

The power converter (1) according to item 2, wherein

the control circuit (13)

controls the fifth switching element (S5) to be in an on state in thethird pattern when the sixth switching element (S6) is controlled to bein an on state in the first pattern, and

controls the eighth switching element (S8) to be in an on state in thethird pattern when the seventh switching element (S7) is controlled tobe in an on state in the first pattern.

This makes it possible to use the upper switching elements (S5, S7) andthe lower switching elements (S6, S8) alternately to short-circuit thesecondary side so that the heat is prevented from being concentrated inthe upper or lower switching elements.

[Item 4]

The power converter (1) according to item 2 or 3, wherein

the control circuit (13)

controls the eighth switching element (S8) or the fifth switchingelement (S5) to be in an on state in the second pattern, and

controls the seventh switching element (S7) or the sixth switchingelement (S6) in an on state in the fourth pattern.

This can reduce conduction loss of the diode by performing synchronousrectification.

[Item 5]

The power converter (1) according to item 3, wherein

the control circuit (13) fixes the phase difference between the firstleg and the second leg and controls a voltage or current of powersupplied from the first DC part (E1, C1) to the second DC part (C2, E2)according to at least one of an on period of the sixth switching element(S6) or the seventh switching element (S7) in the first pattern or an onperiod of the fifth switching element (S5) or the eighth switchingelement (S8) in the third pattern.

This makes it possible to control the voltage or current by controllingthe secondary side without controlling the primary side.

[Item 6]

The power converter (1) according to item 5, wherein

the control circuit (13) sets the phase difference to 0.

This can reduce loss incurred in high-frequency operation.

[Item 7]

The power converter (1) according to any one of items 1 through 6,wherein

the control circuit (13)

turns on the sixth switching element (S6) or the seventh switchingelement (S7) in synchronization with turn-off of the second switchingelement (S2), and

turns on the fifth switching element (S5) or the eighth switchingelement (S8) in synchronization with turn-off of the first switchingelement (S1).

This facilitates ZVS operation.

[Item 8]

The power converter (1) according to item 7, wherein the control circuit(13)

turns on the eighth switching element (S8) or the fifth switchingelement (S5) when a dead time elapses since turn-on of the firstswitching element (S1) or later, and

turns on the seventh switching element (S7) or the sixth switchingelement (S6) when a dead time elapses since turn-on of the secondswitching element (S2) or later.

This can reduce the occurrence of recovery loss.

[Item 9]

The power converter (1) according to item 7 or 8, wherein

the control circuit (13)

turns off the eighth switching element (S8) or the fifth switchingelement (S5) earlier than turn-off of the first switching element (S1)by a dead time, and

turns off the seventh switching element (S7) or the sixth switchingelement (S6) earlier than turn-off of the second switching element (S2)by a dead time.

This can reduce the occurrence of a reactive current from the secondaryside.

[Item 10]

The power converter (1) according to any one of items 1 through 9,wherein

for power transfer from the second DC part (C2, E2) to the first DC part(E1, C1) by stepping up a voltage, the control circuit (13) switches adriving signal supplied to the first switching element (S1)—the fourthswitching element (S4) and a driving signal supplied to the fifthswitching element (S5)—the eighth switching element (S8).

This realizes a highly efficient bidirectional DC-DC converter ofstep-up type in which a reactive current is reduced.

[Item 11]

A power converter (1) including:

a first bridge circuit (11) including a first leg and a second leg, thefirst leg including a first switching element (S1) and a secondswitching element (S2) connected in series, the second leg including athird switching element (S3) and a fourth switching element (S4)connected in series, and the first leg and the second leg beingconnected in parallel to a first DC part (E1, C1);

a second bridge circuit (12) including a third leg and a fourth leg, thethird leg including a fifth switching element (S5) and a sixth switchingelement (S6) connected in series, the fourth leg including a seventhdiode (D7) and an eighth diode (D8) connected in series, and the thirdleg and the fourth leg being connected in parallel to a second DC part(C2, R2);

an insulated transformer (TR1) connected between the first bridgecircuit (11) and the second bridge circuit (12);

a control circuit (13) that controls the first switching element(S1)—the sixth switching element (S6), wherein

diodes (D1-D6) are connected or formed in antiparallel to the firstswitching element (S1)—the sixth switching element (S6), respectively,

the seventh diode (S7) and the eighth diode (S8) are connected in adirection opposite to that of the second DC part (C2, R2), and,

for power transfer from the first DC part (E1, C1) to the second DC part(C2, R2) by stepping up a voltage,

the second bridge circuit (12) includes a period in which a secondarywinding (n2) of the insulated transformer (TR1) and the second DC part(C2, E2) conduct and a period in which ends of the secondary winding(n2) of the insulated transformer (TR1) are short-circuited in thesecond bridge circuit (12),

the control circuit (13)

fixes a phase difference between the first leg and the second leg, and

variably controls a simultaneous off period of the fifth switchingelement (S5) and the sixth switching element (S6).

This realizes a highly efficient unidirectional DC-DC converter ofstep-up type in which the cost is reduced.

INDUSTRIAL APPLICABILITY

The present disclosure is applicable to DAB converters.

REFERENCE SIGNS LIST

E1 first DC power source, E2 second DC power source, 1 power converter,11 first bridge circuit, 12 second bridge circuit, 13 control circuit,S1-S8 switching elements, D1-D8 diodes, L reactor, TR1 insulatedtransformer, n1 primary winding, n2 secondary winding, L1 first leakinductance, L2 second leak inductance, C1 first capacitor, C2 secondcapacitor, R2 load

1. A power converter comprising: a first bridge circuit including afirst leg and a second leg, the first leg including a first switchingelement and a second switching element connected in series, the secondleg including a third switching element and a fourth switching elementconnected in series, and the first leg and the second leg beingconnected in parallel to a first DC part; a second bridge circuitincluding a third leg and a fourth leg, the third leg including a fifthswitching element and a sixth switching element connected in series, thefourth leg including a seventh switching element and an eighth switchingelement connected in series, and the third leg and the fourth leg beingconnected in parallel to a second DC part; an insulated transformerconnected between the first bridge circuit and the second bridgecircuit; a control circuit that controls the first switching element—theeighth switching element, wherein diodes are connected or formed inantiparallel to the first switching element—the eighth switchingelement, respectively, and, for power transfer from the first DC part tothe second DC part by stepping up a voltage, the second bridge circuitincludes a period in which a secondary winding of the insulatedtransformer and the second DC part conduct and a period in which ends ofthe secondary winding of the insulated transformer are short-circuitedin the second bridge circuit, the control circuit fixes a phasedifference between the first leg and the second leg, variably controls asimultaneous off period of the fifth switching element and the sixthswitching element, and variably controls a simultaneous off period ofthe seventh switching element and the eighth switching element.
 2. Thepower converter according to claim 1, wherein for power transfer fromthe first DC part to the second DC part by stepping down a voltage, thecontrol circuit performs control that includes: a first pattern in whichthe first switching element and the fourth switching element are in anon state, the second switching element and the third switching elementare in an off state, and the ends of the secondary winding of theinsulated transformer are short-circuited in the second bridge circuit;a second pattern in which the first switching element and the fourthswitching element are in an on state, the second switching element andthe third switching element are in an off state, and the second bridgecircuit is in a rectification state; a third pattern in which the secondswitching element and the third switching element are in an on state,the first switching element and the fourth switching element are in anoff state, and the ends of the secondary winding of the insulatedtransformer are short-circuited in the second bridge circuit; and afourth pattern in which the second switching element and the thirdswitching element are in an on state, the first switching element andthe fourth switching element are in an off state, and the second bridgecircuit is in a rectification state.
 3. The power converter according toclaim 2, wherein the control circuit controls the fifth switchingelement to be in an on state in the third pattern when the sixthswitching element is controlled to be in an on state in the firstpattern, and controls the eighth switching element to be in an on statein the third pattern when the seventh switching element is controlled tobe in an on state in the first pattern.
 4. The power converter accordingto claim 2 or 3, wherein the control circuit controls the eighthswitching element or the fifth switching element to be in an on state inthe second pattern, and controls the seventh switching element or thesixth switching element in an on state in the fourth pattern.
 5. Thepower converter according to claim 3, wherein the control circuit fixesthe phase difference between the first leg and the second leg andcontrols a voltage or current of power supplied from the first DC partto the second DC part according to at least one of an on period of thesixth switching element or the seventh switching element in the firstpattern or an on period of the fifth switching element or the eighthswitching element in the third pattern.
 6. The power converter accordingto claim 5, wherein the control circuit sets the phase difference to 0.7. The power converter according to any one of claims 1 through 6,wherein the control circuit turns on the sixth switching element or theseventh switching element in synchronization with turn-off of the secondswitching element, and turns on the fifth switching element or theeighth switching element in synchronization with turn-off of the firstswitching element.
 8. The power converter according to claim 7, whereinthe control circuit turns on the eighth switching element or the fifthswitching element when a dead time elapses since turn-on of the firstswitching element or later, and turns on the seventh switching elementor the sixth switching element when a dead time elapses since turn-on ofthe second switching element or later.
 9. The power converter accordingto claim 7 or 8, wherein the control circuit turns off the eighthswitching element or the fifth switching element earlier than turn-offof the first switching element by a dead time, and turns off the seventhswitching element or the sixth switching element earlier than turn-offof the second switching element by a dead time.
 10. The power converteraccording to any one of claims 1 through 9, wherein for power transferfrom the second DC part to the first DC part by stepping up a voltage,the control circuit switches a driving signal supplied to the firstswitching element—the fourth switching element and a driving signalsupplied to the fifth switching element—the eighth switching element.11. A power converter comprising: a first bridge circuit including afirst leg and a second leg, the first leg including a first switchingelement and a second switching element connected in series, the secondleg including a third switching element and a fourth switching elementconnected in series, and the first leg and the second leg beingconnected in parallel to a first DC part; a second bridge circuitincluding a third leg and a fourth leg, the third leg including a fifthswitching element and a sixth switching element connected in series, thefourth leg including a seventh diode and an eighth diode connected inseries, and the third leg and the fourth leg being connected in parallelto a second DC part; an insulated transformer connected between thefirst bridge circuit and the second bridge circuit; a control circuitthat controls the first switching element—the sixth switching element,wherein diodes are connected or formed in antiparallel to the firstswitching element—the sixth switching element, respectively, the seventhdiode and the eighth diode are connected in a direction opposite to thatof the second DC part, and, for power transfer from the first DC part tothe second DC part by stepping up a voltage, the second bridge circuitincludes a period in which a secondary winding of the insulatedtransformer and the second DC part conduct and a period in which ends ofthe secondary winding of the insulated transformer are short-circuitedin the second bridge circuit, the control circuit fixes a phasedifference between the first leg and the second leg, and variablycontrols a simultaneous off period of the fifth switching element andthe sixth switching element.